STUDIES  ON  THE  IREIG&TION  OF 


CITRUS  GROVES. 


Edward  E»  Thornae  «, 


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UNIVERSITY  OF    CALIFORNIA   PUBLICATIONS 


STUDIES  ON  THE  IRRIGATION  OF 
CITRUS  GROVES 


BY 
EDWARD   E.  THOMAS 


BULLETIN  No.  341 

MARCH,  1922 


DIVISION  OF  SUBTROPICAL  HORTICULTURE 

COLLEGE  OF  AGRICULTURE 

BERKELEY,  CALIFORNIA 


UNIVERSITY  OF  CALIFORNIA  PRESS 

BERKELEY,  CALIFORNIA 

1922 


DAVID  P.  BARROWS,  President  of  the  University. 


EXPERIMENT  STATION  STAFF 

HEADS  OF  DIVISIONS 

THOMAS  FORSYTH  HUNT,  Dean. 

EDWARD  J.  WICKSON,  Horticulture  (Emeritus). 

,  Director  of  Resident  Instruction. 

CLARENCE  M.  HARING,  Veterinary  Science,  Director  Agricultural  Experiment 
Station. 

B.  H.  CROCHERON,  Director  of  Agricultural  Extension. 

H.  J.  WEBBER,  Citrieulture,  Director  Citrus  Experiment  Station. 
H.  E.  VAN  NORMAN,  Dairy  Management. 
WILLIAM  A.  SETCHELL,  Botany. 
MYER  E.  JAFFA,  Nutrition. 
RALPH  E.  SMITH,  Plant  Pathology. 
JOHN  W.  GILMORE,  Agronomy. 
CHARLES  F.  SHAW,  Soil  Technology. 
JOHN  W.  GREGG,  Landscape  Gardening  and  Floriculture. 
FREDERIC  T.  BIOLETTI,  Viticulture  and  Fruit  Products. 
WARREN  T.  CLARKE,  Agricultural  Extension. 
ERNEST  B.  BABCOCK,  Genetics. 
GORDON  H.  TRUE,  Animal  Husbandry. 
WALTER  MULFORD,  Forestry. 
JAMES  T.  BARRETT,  Plant  Pathology. 
FRITZ  W.  WOLL,  Animal  Nutrition. 
W.  P.  KELLEY,  Agricultural  Chemistry. 
H.  J.  QUAYLE,  Entomology. 
ELWOOD  MEAD,  Rural  Institutions. 
H.  8.  REED,  Plant  Physiology. 
L.  D.  BATCHELOR,  Orchard  Management. 
J.  C.  WHITTEN,  Pomology. 
*FRANK  ADAMS,  Irrigation  Investigations. 

C.  L.  ROADHOUSE,  Dairy  Industry. 
R.  L.  ADAMS,  Farm  Management. 

W.  B.  HERMS,  Entomology  and  Parasitology. 

F.  L.  GRIFFIN,  Agricultural  Education. 
JOHN  E.  DOUGHERTY,  Poultry  Husbandry. 

D.  R.  HOAGLAND,  Plant  Nutrition. 

G.  H.  HART,  Veterinary  Science. 

L.  J.  FLETCHER,  Agricultural  Engineering. 
EDWIN  C.  VOORHIES,  Assistant  to  the  Dean. 

CITRUS  EXPERIMENT  STATION 
DIVISION   OF  AGRICULTURAL,   CHEMISTRY 
W.  P.  KELLEY  A.  B.  CUMMINS 

E.  E.  THOMAS  S.  M.  BKOWN 


*  In  cooperation  with  office  of  Public  Roads  and  Rural  Engineering,   U.  8.  Department  of 
Agriculture. 


STUDIES  ON  THE  IRRIGATION  OF  CITRUS  GROVES1 

By  EDWAKD  E.  THOMAS 


There  are  many  complex  and  as  yet  unsolved  problems  connected 
with  irrigation  in  semiarid  regions.  As  will  be  shown  later,  the 
method  employed  in  the  application  of  water  may  be  the  deciding 
factor  between  success  and  failure  of  crops.  In  order  to  obtain  the 
best  results,  a  careful  study  of  the  water  requirements  of  the  crops 
grown  and  of  the  best  method  of  applying  the  water  should  be  made. 

Similar  soils  under  different  atmospheric  conditions  may  require 
variations  in  the  irrigation  practice.  The  rates  of  transpiration  by 
the  crop  and  evaporation  from  the  surface  of  the  soil  are  probably 
much  greater  in  the  interior  sections  of  California  than  in  the  coastal 
regions,  because  the  temperature  and  aridity  of  the  former  are 
higher  than  of  the  latter.  These  factors  may  determine,  in  consider- 
able measure,  the  times  of  application  and  the  amount  of  water 
needed. 

Wide  differences  occur  in  the  soil  conditions  in  different  sections. 
The  method  of  irrigation  best  suited  to  heavy  soils  may  differ  from 
that  of  light  sandy  soils,  for.  the  reason  that  the  former  type  tends 
to  restrict  the  movement  of  water,  while  the  latter  permits  freer 
percolation  and  in  turn  gives  up  the  water  more  readily. 

The  content  of  colloidal  material  may  vary  in  soils  which  have 
the  same  wilting  coefficient  as  determined  by  the  centrifuge  method. 
A  soil  which  contains  an  appreciable  amount  of  colloidal  material 
will  swell  or  expand  upon  being  wetted.  This  swelling  tends  to 
retard  the  movement  of  water  and  also  reduces  aeration.  A  differ- 
ence in  the  swelling  may  be  brought  about  not  only  by  a  difference 
in  the  amount  of  colloid  present,  but  also  by  a  difference  in  the  nature 
of  the  colloid.  Consequently,  two  soils,  although  they  may  have  the 
same  water  holding  capacity  as  judged  by  the  centrifuge  method, 
may  act  in  an  entirely  different  manner  when  wetted. 

Generally  speaking,  the  practice  of  irrigation  in  the  citrus  groves 
of  California  has  followed  certain  arbitrary  rules,  especially  in  regard 
to  the  frequency  of  application  and  the  amount  of  water  applied. 
There  is  very  little  definite  knowledge  concerning  the  economy  of 
the  methods  in  use. 


i  Paper  No.  81,  University  of  California  Citrus  Experiment  Station  and  Grad- 
uate School  of  Tropical  Agriculture,  Riverside,  California. 


209942 


354  UNIVERSITY   OF    CALIFORNIA — KXI'KRIMKNT    STATION 

The  fact  that  crop  growth  is  stimulated  by  the  application  of 
moderate  amounts  of  water,  seems  to  have  led  some  citrus  growers 
into  the  error  of  applying  excessive  amounts.  Instances  are  known 
where  as  much  as  sixty  acre- inches  per  acre  have  been  applied  in 
one  irrigation  season.  In  such  cases,  the  naturally  dry  portion  of 
the  year  is  effectively  converted  into  a  wet  season,  so  far  as  the  soil 
is  concerned,  since  the  annual  rainfall  is  only  ten  to  fifteen  inches, 
which  falls  mainly  in  the  winter  months.  Excessive  irrigation  is 
most  prevalent  in  sections  where  the  water  is  drawn  from  readily 
available  supplies,  such  as  rivers. 

There  are  instances  where  water  has  been  applied  early  in  the 
spring,  before  the  usual  irrigation  season,  with  a  view  to  "storing 
up"  moisture  in  the  soil  for  use  during  the  dry  season.  Later, 
whenever  irrigation  water  is  available,  excessive  amounts  may  again 
be  applied,  regardless  of  the  moisture  content  of  the  soil  at  the 
time.  Many  citrus  groves  located  on  heavy  soils  have  been  heavily 
irrigated  as  frequently  as  every  ten  days  to  two  weeks  during  the 
spring  and  early  part  of  the  summer. 

Citrus  trees  are  more  sensitive  to  an  excess  of  moisture  than 
some  other  crops.  For  this  reason  the  supply  of  soil  moisture  in 
citrus  groves  should  be  carefully  controlled. 

The  methods  of  distributing  the  water  have  not  been  given  ade- 
quate consideration.  At  present,  the  irrigation  furrows  are  often 
from  500  to  800  feet  in  length  and  one  citrus  grove  has  been  under 
observation  which  is  irrigated  from  furrows  1380  feet  in  length. 
As  will  be  shown  below,  a  uniform  distribution  of  irrigation  water 
is  impossible  with  the  use  of  furrows  of  such  length. 

An  interesting  experiment  to  test  the  effect  of  various  moisture 
conditions  on  the  growth  of  lemon  trees  was  conducted  with  a  loam 
soil  of  the  Limoneira  Ranch  Company,  Santa  Paula,  California,  by 
Fowler  and  Lipman.2  By  growing  young  lemon  trees  in  cylinders 
in  which  the  moisture  content  was  varied,  they  determined  the  per- 
centage of  moisture  at  which  the  soil  must  be  kept  in  order  to  pro- 
duce the  best  growth.  They  found  that  the  growth  of  the  trees  was 
retarded  when  the  moisture  content  was  either  above  or  below  the 
optimum.  The  optimum,  however,  appears  to  be  not  a  fixi-d  point. 
but  rather  covers  a  considerable  range. 

Not  all  of  the  moisture  in  a  soil  can  be  absorbed  by  plants.  A 
part  of  it  is  unavailable,  the  amount  of  which  varies  with  the  soil 


2  "Optimum  Moisture  Condition*  for  Young  Lemon  Trees  on  a  Loam  Soil." 
By  L.  W.  Fowler  and  C.  B.  Lipman,  Univ.  of  California  Publication*  in  Ayri 
cultural  Science*,  Vol.  3,  No.  2,  pp.  25-36.  Heptemlxr,  1917. 


BULLETIN  341]        STUDIES  ON  THE  IRRIGATION  OF  CITRUS  GROVES  355 

type.  The  amount  of  unavailable  moisture  in  a  heavy  soil  is  suffi- 
cient to  saturate  a  light,  sandy  soil.  It  is  important,  therefore,  to 
distinguish  between  the  available  and  unavailable  moisture  in  a  soil. 

Briggs  and  Shantz  state  that,3  "in  connection  with  the  study  of 
the  moisture  requirements  of  plants  in  semiarid  regions  it  is  neces- 
sary to  be  able  to  determine  quickly  the  soil-moisture  content  at  the 
wilting  point.  This  constitutes  the  datum  from  which  the  moisture 
available  for  growth  can  be  calculated  and  without  which  field  de- 
terminations of  soil  moisture  are  of  little  value." 

The  unavailable  water  in  soils  is  now  commonly  determined  by 
the  use  of  the  moisture-equivalent  centrifuge.  The  data  thus  ob- 
tained are  referred  to  as  "wilting  coefficients."  The  wilting  co- 
efficient is  supposed  to  indicate  the  moisture  content  of  the  soil  when 
a  plant  becomes  permanently  wilted.  By  the  permanent  wilting  of 
the  plant,  is  meant  a  condition  from  which  it  cannot  regain  its  turgor 
when  surrounded  by  a  saturated  atmosphere,  unless  water  be  added 
to  the  soil. 

As  previously  intimated,  the  determination  of  the  so-called  wilt- 
ing coefficient  may  not  always  give  data  by  which  the  water-absorb- 
ing and  water-retaining  power  of  soils  derived  from  entirely  dif- 
ferent sources  can  be  measured  accurately.  Such  data,  however, 
probably  afford  information  of  considerable  value  in  comparing  soils 
of  a  similar  type. 

The  present  paper  is  based  on  experiments  which-  were  conducted 
for  two  seasons  in  certain  citrus  groves  located  on  a  heavy  type  of 
soil.  The  groves  studied  are  located  in  the  Whittier,  East  Whittier 
and  La  Habra  sections  of  Los  Angeles  and  Orange  counties.  "While 
the  results  obtained  and  the  conclusions  drawn  may  not  apply  equally 
to  other  types  of  soil,  it  is  believed  that  some  of  the  practical  lessons 
taught  by  these  studies  are  widely  applicable,  and  sufficiently  gen- 
eral to  warrant  publication. 

This  investigation  was  conducted  during  the  irrigation  seasons 
of  1916  and  1917.  The  soils  studied  were  all  of  the  same  general 
type,  having  a  wilting  coefficient  of  from  9  to  19  per  cent,  with  an 
average  of  14  per  cent. 

Four  points  have  been  investigated  in  this  work:  (1)  the  amount 
of  water  in  the  soil  at  the  upper  and  lower  ends  of  long  furrows; 
(2)  the  appearance  and  productiveness  of  the  trees  growing  at  the 
upper  and  lower  ends  of  long  furrows;  (3)  the  amount  of  available 


3  "The  Wilting  Coefficient  of  different  plants  and  its  indirect  determina- 
tion. ' '  By  Lyman  J.  Briggs  and  H.  L.  Shantz.  Bureau  of  Plant  Industry, 
Bull.  No.  230,  p.  8,  February,  1912. 

DIVISION  OF  SUBTROPICAL  HORTICULTURE 
COLLEGE  OF  AGRICULTURE 


356  UNIVERSITY   OF    CALIFORNIA — EXPERIMENT    STATION 

moisture4  in  the  soil  when  the  water  was  applied  at  intervals  of  30, 
45  and  60  days;  and  (4)  the  effect  of  deep  and  shallow  cultivation 
on  the  water-absorbing  power  of  the  soil. 

Altogether,  twenty  different  groves  have  been  studied.  The  re- 
sults obtained  with  only  seven  of  these,  chosen  to  represent  the  dis- 
tricts in  question,  will  be  presented  at  present.5 

The  same  general  course  of  procedure  was  followed  in  each  grove. 
Samples  of  soil  were  taken  at  regular  intervals  from  a  given  plot  and 
a  determination  was  made  of  the  total  moisture  content  in  each 
foot  to  a  depth  of  four  feet  and,  in  some  instances,  to  a  depth  of 
six  or  eight  feet. 

The  wilting  coefficient  of  samples  representing  each  foot  of  the 
plots  studied  was  determined  by  Mr.  C.  A.  Jensen,  formerly  of  the 
United  States  Department  of  Agriculture,  to  whom  acknowledge- 
ment and  thanks  are  extended. 

I.  Comparison  of  the  amount  of  water  in  the  soil  at  the  upper  and 
lower  ends  of  long  furrows. 

In  the  grove  chosen  to  show  the  effect  of  the  length  of  furrow  on 
the  content  of  water  in  the  soil,  the  furrows  were  812  feet  long  with 
a  fall  of  sixteen  feet.  Soil-moisture  determinations  were  made  on 
two  plots  in  this  grove  during  the  1917  irrigation  season.  The  wilt- 
ing coefficient  of  the  soil  to  a  depth  of  eight  feet,  is  given  in  table  I. 

TABLE  I 
THE  WILTING  COEFFICIENT  OF  PLOTS  A  AND  B,  GROVE  1 

Plot         1st  ft.    2d  ft.    3d  ft.    4th  ft.   5th  ft.   6th  ft.   7th  ft.   8th  ft. 

A1 16.90%    17.15%    17.30%   16.90%    16.75%    16.75%    18.30%    15.20% 

B* 16.90%   17.20%    16.90%    18.80%    19.00%    17.60%    16.80%    16.60% 

'Situated  at  the  upper  end  of  the  irrigation  furrows. 
'Situated  near  the  lower  end  of  the  irrigation  furrows. 

As  is  common  in  irrigation  practice  in  certain  localities,  the 
owner  of  this  grove  irrigates  whenever  he  considers  it  necessary 
rather  than  at  any  stated  interval.  During  the  season  of  1917  four 
irrigations  were  applied  at  intervals  of  thirty  days,  followed  by  three 
additional  irrigations  at  intervals  of  25,  37  and  41  days,  respectively, 


4  The  term  "available  moisture,"  as  use<l  in  this  paper,  signifies  the  amount 
of  moisture  that  is  in   excess  of  the  amount  of  moisture   represented   by  the 
wilting  coefficient. 

5  See  a  previous  paper  by  the  writer,  "Irrigation  Stuilic.s  at  Whittier. "     The 
California  Citrograph,  January-February,  1917. 


BULLETIN  341]        STUDIES  ON  THE  IRRIGATION  OF  CITRUS  GROVES  357 

making  a  total  of  seven  applications  for  the  season.  The  first  appli- 
cation was  made  on  April  10  and  the  last  on  October  23.  A  total 
of  18.5  acre-inches  of  water  was  applied  per  acre. 

When  we  study  the  data  showing  the  moisture  found  in  the 
two  plots  of  this  grove  (table  II)  the  one  (A)  located  from  twenty- 
two  to  sixty-two  feet  from  the  point  of  distribution,  and  the  other 
(B)  located  621  to  661  feet  from  the  distributing  line,  we  find  that 
instead  of  the  water  being  evenly  distributed,  the  soil  of  plot  B  con- 
tained much  less  moisture  than  that  of  plot  A.  Since  all  of  the 
water  was  distributed  from  the  same  pipe  line,  it  was  not  possible 
to  determine  the  amount  of  the  water  that  was  actually  delivered  to 
each  plot. 

The  data  in  table  II  shows  that  the  subsoil  in  plot  A  also  con- 
tained much  more  moisture  than  did  the  subsoil  in  plot  B.  Since 
the  roots  of  citrus  trees  seldom  penetrate  below  a  depth  of  four 
feet  in  this  type  of  soil,  a  considerable  part^of  the  water  applied  to 
plot  A  certainly  penetrated  below  the  reach  of  the  roots  and,  there- 
fore, did  not  benefit  the  trees.  Soluble  plant  food  must  also  have 
been  leached  below  the  reach  of  the  roots  of  the  trees. 

These  data  indicate  that,  under  the  present  system  of  furrow 
irrigation,  a  uniform  distribution  of  water  throughout  a  grove  cannot 
be  accomplished  with  the  use  of  long  furrows.  With  this  method 
of  application,  a  portion  of  the  water  that  is  applied  must  inevitably 
be  wasted,  because  of  too  deep  penetration  near  the  upper  end  of 
the  furrows.  In  sections  where  the  water  is  obtained  from  wells  or 
expensive  irrigation  systems,  this  waste  adds  materially  to  the  oper- 
ating expenses  of  the  grove.  On  account  of  leaching,  soluble  plant 
food  is  carried  below  the  root  zone  of  the  trees,  and  increases  the 
expense  for  fertilizers.  As  will  be  shown  later,  excessive  irrigation 
may  injure  orange  trees. 

These  faults  might  be  corrected  by  increasing  the  number  of 
distributing  lines.  The  length  of  the  furrows  should  not  exceed  250 
to  300  feet. 

A  portion  of  the  expense  incident  to  the  installation  of  extra 
pipe  lines  would  be  offset  at  once  by  the  reduction  in  the  amount 
of  water  necessary  to  be  applied,  and,  as  shown  below,  the  produc- 
tivity of  the  trees  would  be  increased. 


358 


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BULLETIN  341 J         STUDIES  ON  THE   IRRIGATION   OF  CITRUS  GROVES  359 

II.  A  comparison  of  the  appearance  and  productiveness  of  trees  at 
the  upper  and  lower  ends  of  long  furrows. 

Two  plots,  K  and  L,  (grove  2)  are  chosen  to  illustrate  the  effects 
of  excessive  irrigation  on  the  appearance  and  productivity  of  the 
trees.  This  grove  was  irrigated  from  four  distributing  lines  and 
plot  K  was  located  at  the  lower  end  of  the  furrows  in  an  upper 
block  of  trees,  while  plot  L  was  located  just  across  a  distributing 
pipe  line  from  plot  K  at  the  upper  end  of  the  furrows  in  the  sec- 
ond block  of  trees.  Each  plot  consisted  of  nine  orange  trees.  Plot 
K  being  adjacent  to  plot  L,  it  is  believed  that  the  soil  and  climatic 
conditions  in  the  two  plots  were  similar  in  all  respects. 

The  entire  grove  of  sixty-seven  acres  was  given  the  same  kind 
of  cultivation  and  fertilization,  and  it  was  irrigated  throughout  at 
thirty-day  intervals,  beginning  the  latter  part  of  April. 

The  amount  of  water  delivered  to  plots  K  and  L  was  very  dif- 
ferent, as  may  be  appreciated  when  their  location  is  considered. 
Plot  K  was  located  at  the  lower  end  of  irrigation  furrows  248  feet 
in  length,  and  plot  L  was  located  just  across  a  distributing  pipe 
line  from  plot  K  at  the  upper  end  of  furrows  528  feet  long.  The 
entire  section  in  which  plot  K  was  situated  received  an  average  of 
2.43  acre-inches  of  water  per  acre  at  each  irrigation/ but  as  the  ex- 
perimental plot  was  located  at  the  lower  end  of  the  irrigation  fur- 
rows, the  amount  of  water  actually  delivered  to  it  must  have  been 
below  the  average  for  the  plot.  The  section  in  which  plot  L  was 
located  received  an  average  of  4.28  acre-inches  at  each  irrigation. 
Since  plot  L  was  located  at  the  upper  end  of  the  irrigation  fur- 
rows, it  must  have  received  more  water  than  the  average  for  the 
section. 

The  difference  in  the  appearance  of  the  trees  in  different  parts 
of  the  grove  was  very  noticeable.  All  of  the  trees  in  plot  K,  the 
lightly  irrigated  plot,  were  in  fine  condition.  They  bore  an  abund- 
ance of  normal  green  leaves  and,  without  exception,  produced  a 
good  crop  of  fruit.  The  trees  in  plot  L,  on  the  other  hand,  were  in 
poor  condition.  Many  of  the  leaves  dropped  prematurely  and  a 
large  percentage  of  those  remaining  on  the  tree  were  yellow.  The 
trees  appeared  to  be  suffering  from  a  lack  of  nourishment  and  the 
crop  of  fruit  was  light. 

A  careful  record  of  the  amount  of  fruit  produced  on  plots  K 
and  L  was  kept  in  1917.  It  was  found  that  the  nine  trees  in  plot 
K  which  received  the  smaller  amount  of  water  produced  fifty-five 
boxes  of  fruit,  while  those  in  plot  L  yielded  twenty-four  boxes. 

DIVISION  OF  SUBTROPICAL  HORTICULTURE 
COLLEGE  OF  AGRICULTURE 


360 


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BULLETIN  341]        STUDIES  ON  THE  IRRIGATION  OF  CITRUS  GROVES  361 

Thus  the  trees  in  the  adjoining  plots  K  and  L,  with  the  same 
care  and  other  conditions,  with  the  exception  of  the  amount  of 
water  delivered  to  them,  were  totally  unlike  in  appearance  and  pro- 
duction. (The  available  moisture  in  plots  K  and  L,  Grove  2,  is 
given  in  table  III.) 

It  seems  safe  to  conclude  from  the  facts  above  presented,  that 
the  trees  at  the  upper  end  of  the  long  irrigation  furrows  received 
too  much  water.  The  more  or  less  constantly  water-logged  condi- 
tion of  the  soil  in  this  case  must  have  interfered  with  its  proper 
aeration  and  also  must  have  hindered  normal  root  development. 

III.  A  comparison  of  the  amount  of  available  moisture  in  the  soil  when 
irrigations  were  applied  at  intervals  of  30,  45  and  60  days. 

Whenever  possible,  irrigation  practices  should  be  so  arranged 
that  the  water  may  be  applied  to  the  soil  at  the  time  when  needed. 
The  time  of  application,  as  well  as  the  amount  of  water  applied, 
should  be  controlled  by  soil  moisture  tests.  While  this  means  of 
irrigation  control  may  be  difficult  of  execution  where  the  water  is 
obtained  from  irrigation  companies  that  use  the  present  system  of 
water  distribution,  it  is  possible  to  change  the  irrigation  interval, 
and  this  should  be  done  wherever  the  prevailing  system  is  unduly 
faulty.  It,  therefore,  becomes  important  to  know  whether  the  in- 
terval now  in  use  in  a  given  case  is  best  suited  to  the  soil  type  in 
question.  Most  of  the  groves  in  the  district  under  consideration 
are  irrigated  every  thirty  days. 

As  a  means  of  studying  this  question  a  comparison  of  the  avail- 
able moisture  was  made  in  groves  that  were  irrigated  at  intervals  of 
30,  45  and  60  days. 

Two  adjacent  groves  were  studied,  one  of  which,  grove  3,  was 
irrigated  at  intervals  of  thirty  days  and  the  other,  grove  4,  at  in- 
tervals of  sixty  days.  These  two  groves  were  under  the  same  man- 
agement and,  with  the  exception  of  the  interval  between  irriga- 
tions, were  given  the  same  cultural  treatment. 

Grove  4  received  an  average  of  5.7  acre-inches  of  water  per  acre 
every  sixty  days,  which  provides  for  a  lesser  amount  of  water  for 
the  season  as  a  whole,  than  was  applied  to  grove  3,  which  received 
an  average  of  4.28  acre-inches  of  water  per  acre  every  thirty  days. 
Comparisons  of  the  available  moisture  between  plot  C  (grove  3)  and 
plot  G  (grove  4)  are  shown  in  table  IV.  The  wilting  coefficients  of 

these  two  soils  are  as  follows : 

1st  ft.  2d  ft.  3d  ft.  4th  ft. 

Grove  3,  plot  C 14.4%  15.2%  14.9%  14.4% 

Grove  4,  plot  G 13.6%  14.7%  19.9%  14.5% 

DIVISION  OF  SUBTROPICAL  HORT!CULUJR€ 


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BULLETIN  341]        STUDIES  ON  THE  IRRIGATION  OF  CITRUS  GROVES 


363 


The  data  show  that  plot  G,  grove  4,  to  which  the  water  was  ap- 
plied at  sixty-day  irrigation  intervals,  contained  as  much  available 
moisture  as  plot  C,  grove  3,  which  received  a  greater  amount  of 
water  during  the  season,  but  applied  at  thirty-day  intervals.  A 
comparison  of  the  trees  in  the  two  groves  showed  that,  on  the  whole, 
the  trees  in  the  grove  irrigated  at  sixty-day  intervals  were  in  better 
condition  than  those  in  the  grove  irrigated  at  thirty-day  intervals. 

Studies  were  also  made  on  another  grove  (5).  This  grove  was 
divided  into  nine  plots,  A  to  I  inclusive.  During  the  1917  irriga- 
tion season,  plots  A  and  E  were  irrigated  every  sixty  days;  B,  C,  F 
and  G  every  thirty  days;  and  D,  H  and  I  every  forty-five  days. 
At  each  irrigation,  plots  A,  B,  E  and  F  received  an  average  of  four 
acre-inches  per  acre ;  plots  C  and  G  an  average  of  two  acre-inches  per 
acre;  plots  D  and  H  an  average  of  three  acre-inches  per  acre;  and 
plot  I  an  average  of  six  acre-inches  per  acre.  Thus,  plots  A,  C,  D, 
E,  G  and  H  received  the  same  amount  of  water  during  the  season, 
while  plots  B,  F  and  I  received  twice  that  amount.  In  this  grove 
the  irrigation  furrows  were  250  feet  in  length  and  the  plots  were 
all  located  between  forty  and  sixty  feet  from  the  distributing  pipe 
line. 

The  wilting  coefficients  in  the  nine  plots  of  grove  5  are  given  in 
table  V.  A  study  of  these  data  shows  that  the  soil  to  a  depth  of 
eight  feet,  is  quite  uniform  in  all  of  the  plots. 


TABLE  V 
THE  WILTING  COEFFICIENT  IN  THE  PLOTS  IN  GROVE  5 


Plot 


1st  ft.         2d  ft. 


3d  ft.       4th  ft.        5th  ft.        6th  ft.        7th  ft.         8th  ft. 


A 14.06%    13.5%     12.5%     11.0%     12.65%    14.9%      13.8%     14.24% 


B     . 

14.9 

14.1 

14.3 

14.1 

15.3 

16  15 

14.1 

12  2 

C 

15.2 

14.9 

15.0 

15.3 

15.5 

146 

13.5 

127 

D  

...    14.1 

14.6 

13.4 

13.3 

13.85 

13.6 

13.75 

12.45 

E  

14.6 

15.1 

14.9 

15.3 

15.05 

14.5 

13.6 

13.15 

F 

13.0 

12.6 

9.5 

12.8 

156 

155 

155 

150 

G 

13.45 

13.6 

13.85 

14.1 

14.9 

14.5 

13.3 

128 

H  

...      13.05 

14.0 

14.5 

15.2 

15.0 

13.6 

12.9 

11.6 

I... 

13.4 

14.1 

14.7 

15.9 

12.05 

11.8 

10.7 

10.5 

The  data  in  table  VI  show,  that  the  available  moisture  was  more 
uniformly  distributed,  to  a  depth  of  eight  feet,  in  the  plot  which 
was  irrigated  at  sixty-day  intervals  than  in  the  plots  which  were 
irrigated  more  frequently. 


OF  SUBTROPICAL  HOR1ICUL1UI* 
OF  AGRICULTURE 


UNIVERSITY   OF    CALIFORNIA — EXPERIMENT    STATION 


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BULLETIN  341]        STUDIES  ON  THE  IRRIGATION  OP  CITRUS  GROVES 


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,.»v.SION  OF  SUBTROPICAL  HORTICULlUi:! 
COLLEGE  OF  AGRICULTURE 


366  I'NIVERSITY    OF    CALIFORNIA — KXI'KRJMKNT    STATION 

The  data  also  indicate  that  all  of  the  plots  probably  received 
more  water  than  was  essential  for  the  best  development  of  the  trees, 
since  the  available  moisture  in  all  of  the  plots  was  comparatively 
high  throughout  the  season. 

IV.  A  comparison  of  the  effect  of  deep  and  shallow  cultivation  on 
the   water-absorbing  power  of  the  soil. 

Studies  have  also  been  made  on  the  water-absorbing  power  of 
heavy  soil  as  influenced  by  deep  and  shallow  cultivation.  The  groves 
studied  (6  and  7)  are  adjacent  and  the  soil  is  of  a  heavy  type.  Each 
of  these  groves  was  irrigated  at  sixty-day  intervals  in  1916  and  1917. 
Grove  6,  however,  was  irrigated  from  two  pipe  lines,  the  upper  fur- 
rows having  a  length  of  339  feet  and  the  lower  ones  300  feet,  while 
grove  7  is  provided  with  but  one  pipe  line  at  the  upper  end  of  the 
grove  and  the  furrows  are  714  feet  in  length. 

After  the  soil  in  grove  6  had  become  sufficiently  dry  near  the 
surface  to  prevent  puddling,  it  was  harrowed  lightly.  Later,  when 
the  soil  had  dried  out  more  deeply,  a  cultivation  to  a  depth  of  seven 
to  nine  inches  was  given  the  grove.  Usually  this  latter  cultivation 
was  not  made  until  twenty-five  or  thirty  days  after  an  irrigation." 

The  treatment  of  the  soil  in  grove  7  was  very  different  from 
that  just  described.  In  this  case  the  soil  was  cultivated  to  a  depth 
of  three  to  five  inches  at  a  time  when  portions  of  it  were  so  wet  that 
puddling  resulted.  Later  two  or  three  "cultivations  of  a  similar 
nature  were  given  before  the  next  irrigation. 

The  soil  in  grove  7  did  not  absorb  water  as  well  as  the  soil  in 
grove  6.  -This  was  clearly  shown  during  the  winters  of  1916-17  and 
1917-18,  when  a  portion  of  the  water  from  the  rains  ran  off  and 
was  lost,  while  the  soil  in  grove  6  absorbed  all  of  the  rain  water. 

Stable  manure  had  been  applied  to  both  groves,  but  on  account 
of  the  difference  in  cultivation,  it  was  more  thoroughly  incorpor- 
ated with  the  soil  in  the  grove  that  was  tilled  the  more  deeply.  This 
fact  probably  influenced  the  evaporation  of  the  soil  moisture  and 
also  tended  to  increase  the  water-absorbing  power  of  the  soil.  How- 
ever, by  far  the  greatest  difference  in  the  treatment  of  these  soils 
was  in  the  manner  and  depth  of  cultivation. 


•It  is  necessary  to  delay  the  cultivation  of  this  soil  much  longer  than  with 
lighter  types.  Thin  is  due  to  the  fact  that  being  a  heavy  soil  it  absorbs  much 
more  water  than  a  lighter  type,  and  more  time  is  required  for  evaporation  and 
percolation  to  reduce  the  moisture  to  a  point  where  the  soil  can  be  thoroughly 
cultivated  without  injury  from  puddling. 


BULLETIN  341]      STUDIES  ON  THE  IRRIGATION  OF  CITRUS  GROVES  367 

A  comparison  of  the  available  moisture  (table  VII)  shows  that 
on  May  5,  before  the  first  irrigation,  the  soil  in  plots  A  and  B, 
grove  6,  which  was  cultivated  more  judiciously,  contained  greater 
amounts  of  available  moisture  than  that  in  plots  C  and  D,  grove  7. 
This  difference  became  greater  as  the  season  advanced,  until  we 
find  that  in  plot  D,  grove  7,  which  received  the  shallow  cultivation, 
the  moisture  in  the  third  and  fourth  feet  of  soil  was  below  the  wilt- 
ing coefficient  continuously  after  July  5,  and  the  moisture  in  the 
first  and  second  feet  was  below  the  wilting  coefficient  a  portion  of 
the  time.  The  trees  on  this  plot  suffered  from  a  lack  of  moisture, 
and  the  fruit,  as  well  as  the  leaves  of  the  trees,  became  wilted.  On 
the  other  hand,  all  of  the  trees  in  grove  6  were  in  fine  condition 
and  yielded  a  large  amount  of  good  fruit. 

The  great  difference  in  the  physical  condition  and  the  moisture 
content  of  the  soil  of  these  two  groves  clearly  illustrates  the  need  of 
great  care  in  the  cultivation  of  a  heavy  type  of  soil.  The  soil  should 
not  be  stirred  while  it  is  wet,  but  should  be  allowed  to  remain  undis- 
turbed until  sufficiently  dry  to  permit  thorough  and  deep  cultiva- 
tion. 

CONCLUSIONS. 

(1)  The  results  of  this  investigation  indicate  that  irrigation  fur- 
rows which  exceed  250  or  300  feet  in  length  are  undesirable.    When 
furrows  of  a  greater  length  are  used,  the  soil  near  the  upper  end 
of  the  furrows  must  inevitably  receive  an  excess  of  water.     The  re- 
sult is  that  it  becomes  "water-logged."     A  portion  of  the  soluble 
plant  food  will  be  leached  and  carried  below  the  root  zone  by  the 
water. 

(2)  Citrus  trees  growing  on  heavy  soil  may  become  stunted  when 
excessively  irrigated;  the  leaves  turn  more  or  less  yellow  and  many 
of  them  fall  prematurely.    The  yield  of  fruit  is  also  greatly  impaired 
and  in  some  instances  the  trees  may  become  unprofitable.     In  many 
groves  it  would  be  advisable  to  install  a  greater  number  of  distrib- 
uting pipe  lines  in  order  to  shorten  the  irrigation  runs. 

(3)  In  order  to  secure  the  best  results  from  the  application  of 
irrigation  water,  it  should  be  applied  at  the  time  when  it  is  needed, 
as  gauged  by  the  moisture  content  of  the  soil.     Whenever  this  pro- 
cedure is  impracticable,  the  irrigation  water  should  be  applied  at 
intervals  best  suited  to  the  soil  type,  as  determined  by  experience 
or  definite  experiments. 

DIVISION  OF  SUBTROPICAL  HORTICULTURE 

COLLEGE  OF  AGRICULTURE 

BERKELEY,  CALIFORNIA 


368 


UNIVERSITY   OF    CALIFORNIA — EXPERIMENT    STATION 


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BULLETIN  341]         STUDIES  ON  THE  IRRIGATION  OF  CITRUS  GROVES 


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370  UNIVERSITY   OF    CALIFORNIA — EXPERIMENT    STATION 

On  a  heavy  loam  soil  such  as  that  under  consideration,  the  soil 
moisture  was  found  to  remain  more  uniform  and  the  conditions  for 
root  development  better,  with  a  sixty-day  interval  between  irriga- 
tions, than  with  a  thirty-day  interval. 

(4)  The  water-absorbing  power  of  heavy  soil  is  influenced  by 
the  method  of  tillage.  The  soil  should  not  be  stirred  while  it  is  wet. 
Frequent,  shallow  cultivation  tends  to  pack  the  soil  immediately  be- 
low the  cultivated  area.  The  plow-sole  thus  formed  retards  the 
movement  of  water  in  the  soil. 

Good  conditions  for  plant  growth  can  be  obtained  by  harrowing 
the  soil  lightly  after  it  has  become  sufficiently  dry  near  the  surface 
to  prevent  puddling;  then  allowing  it  to  remain  undisturbed  until 
it  has  dried  out  more  deeply  so  as  to  permit  deep  and  thorough  cul- 
tivation.7 


?  The  writer  wishes  to  acknowledge  his  indebtedness  to  Dr.  W.  P.  Kelley  of 
the  Citrus  Experiment  Station  for  many  valuable  suggestions. 


DIV.S.ON 

ourtoF 

ORNIA 


This  book  is  DUE  on  the  last  date  stam 


SB 

3*9 

T3*s 


UC  SOUTHERN  REGIONAL  LIBRARY  FAdUTY 


A    001  095  395    8 


